Hallux Rigidus (HR) is the end stage of great toe osteoarthritis, which is a degenerative joint disease that effects approximately 1 in 40 people over the age of 50. The development of arthritis in the first metatarsophalangeal joint (MTPJ) is painful, effecting quality of life, reducing mobility and altering gait patterns. Early treatments for pain relief include physical therapy, pharmaceuticals and modified footwear. If these treatments fail to provide relief and the arthritis progresses, joint function is largely lost (hence rigidus for end-stage disease progression). Concerning veterans, from 2008-2017, nationwide data from the Veterans Healthcare System yielded 87,798 patient visits for HR (1,897 in that same period for the VA Puget Sound Healthcare System), and the number of patient visits has consistently increased year to year from 9,396 visits nationwide in 2008 to 22,180 visits in 2017. In order to properly match veteran HR patients with proper treatment, the stage of the disease progression needs to be properly classified. The most common classification systems use radiographic findings. Typically, AP and lateral X-ray views are evaluated for joint space (reduced space indicates lost cartilage), osteophytes (and indicator of arthritis), and joint shape (as HR patients preferentially lose cartilage and gain osteophytes on the dorsal surface). Yet the exact features evaluated, and how they are considered, is not standardized. This issue is further complicated by some grading schemes which blend clinical findings of range of motion and pain, into the already-complex field of radiographic features. Finally, while 2D X-rays are the standard for diagnosis, HR is a complex 3D disease and it has been hypothesized that certain bone positional (metatarsus primus elevatus) and morphological (articular surface shape) may be correlated with HR and possibly predispose patients to develop HR. Such measurements can be difficult to realize on 2D X-ray, and using manual and subjective measures. Therefore we propose to utilize existing weight-bearing CT data from control feet and obtain similar data from fifteen HR subjects. We will use this imaging data to evaluate the Intraclass Correlation associated with some of the more common HR classification schemes (Aim 1). This will allow us to establish reliability measures for the existing methods of clinically grading hallux rigidus, this will aid to inform the clinical community of the reliability of currently practiced diagnostic methods?which have a direct impact on treatment selection and patient care. We will also use our prior experience in evaluating 3D bony anatomy to semi-automate a suite of first ray positional (joint spacing, first metatarsal alignment, metatarsus primus elevatus) and morphological (sagittal plane metatarsal head curvature) features, which are suspected to have a correlation to HR (Aim 2). One of the greatest challenges associated with this work relate to developing a user-friendly environment by which clinicians can rapidly select bone features, which will algorithmically determined measurements for immediate reporting. To this end we will hire a research engineer with specific background in developing user interfaces and 3D digital object manipulation. This specialization is designed to generate a well-documented and polished analysis program which can be readily learned by clinicians. The other significant challenge is in determining the potential benefit of our proposed additional measurements in either enhancing or replacing the current classifications schemes. The benefit of these measurements will support our additional goal to lay the groundwork for either enhancing a current classification scheme (potentially by adding an additional measure) or developing a new classification scheme (via a new slate of semi-automated measurements) two project areas which this study will develop preliminary data for follow up with a future Merit Review.